The present invention relates to the field of optical shaft encoders, and more particularly to a method and apparatus for detecting tampering with a meter having an encoded register display.
Optical shaft encoders are well known for their use in determining the angular position of a rotating shaft. Such optical shaft encoders are used in various types of machinery and machine tools where information concerning the precise angular relationship between a shaft and another component is needed.
Optical shaft encoders generally consist of an optical shutter, such as a disk or drum, which is rigidly attached to the shaft whose position is to be determined. The optical shutter is used to modulate the transmission of light between a light emitter (e.g. a light emitting diode) and a light detector (e.g. a photodiode or phototransistor). This modulation may be transmissive, in which case the optical shutter has a pattern of slots formed on its surface and the light emitter and light detector are arranged on opposite sides of the optical shutter. Alternatively, the modulation may be reflective, in which case the optical shutter has a pattern of reflective (e.g. white) and absorptive (e.g. black) areas on its surface and the light emitter and light detector are arranged adjacent one another on the same side of the optical shutter.
In general, the resolution of a optical shaft encoder depends on the number of light emitting and light detecting devices used. For example, using one emitter and one detector, whose output will be either a "1" (on) or "0" (off), will give an angular resolution of 1/2 a revolution of the shaft. If two emitters and two detectors are used, the detector outputs can have up to four different states (e.g. 00, 01, 10, or 11) and thus will be able to resolve 1/4 of a revolution. Three emitters and three detectors can have eight different states and resolve 1/8 of a revolution; four emitters and four detectors can have sixteen different states and resolve 1/16 of a revolution, etc.
In co-pending application Ser. No. 650,031, filed Sept. 13, 1984, entitled "Optical Shaft Encoder" an optical shaft encoder is disclosed which utilizes an optical shutter, such as a disk or drum, mounted to a shaft and having a pattern of light transmissive and light reflective/absorptive areas formed thereon. A first light emitter and a first light detector are arranged on one side of the optical shutter and a second light emitter and a second light detector are arranged on the opposite side of the optical shutter facing the first light emitter and detector. Through an appropriate choice of the patterns of light transmissive and reflective/absorptive areas formed on the optical shutter, and by alternately energizing the light emitters, up to 16 different angular positions of the optical shutter and shaft with respect to the light emitters and light detectors may be resolved.
Optical shafts encoders of the foregoing types have been proposed for use in detecting the position of the shafts which drive a clock-dial type register display such as used in a gas, water, or electricity meter. Such a display usually consists of a dial face similar to that used on a clock and having the numerals 0-9 printed thereon. A pointer is removably attached to each shaft, and each shaft is in turn linked to the metering mechanism of the meter. The shaft and pointer are normally rotated by an amount that is proportional to the amount of billable commodity (e.g. gas, water, electricity) being monitored by the meter.
One problem often encountered with such metering devices is that the register display can be easily tampered with in order to give a false reading of the amount of the commodity which has been measured by the meter. Such tampering can be carried out simply by moving the dial pointers into new positions while the shaft positions remain unchanged. Also, in some types of electricity meters, the meter can be caused to run backwards by simply removing the meter from its socket and replacing it upside-down for a certain period of time. The tamperer later returns the meter to its normal, upright position. The register display runs backwards while upside-down and thus will show a lower amount of electrical power energy being consumed than is actually the case.